A large number of individuals are afflicted with disorders of cartilage or cartilage-like tissues, and hence, the makeup of normal and abnormal cartilage tissue as well as the breakdown of cartilage tissue is an important topic of current investigations. Osteoarthritis is a condition characterized by the breakdown of cartilage tissue and occurs in the majority of individuals over the age of 55. One of the most important causes of absenteeism from the workplace is chronic back pain, which in many cases is related to deterioration of the discs which are between the vertebrae and which are formed of tissue that is closely related to cartilage. Another tissue that is related to cartilage is the tissue that comprises the cornea of the eye, and there are indications that the cornea-clouding, blindness-causing disease, macular dystrophy, results from the inability of certain individuals to biosynthesize the macromolecules present in normal corneal tissue.
Diagnosis of cartilage, disc, or corneal disorders is often only detected after the disorders have reached relatively advanced stages where the individual experiences pain in the joints or the back, or, in the case of macular dystrophy, is beginning to lose his sight. It would be desirable to have a method of diagnosing the onset of cartilage, disc or corneal tissue disorders before they become symptomatic.
Proteoglycans are major components of the extracellular matrix of cartilage where they are found largely organized into aggregates. These aggregates contain proteoglycan molecules which can specifically bind to hyaluronic acid through a portion of the molecule termed the hyaluronic acid-binding region.
A cartilage tissue proteoglycan molecule has a core protein backbone of approximately 2000-4000 amino acid residues. One end of this core protein contains N-linked oligosaccharides and is able to interact very specifically with hyaluronic acid. It is termed the hyaluronic acid-binding region. The major portion of the protein moiety is rich in covalently attached glycosaminoglycan moieties, particularly keratan sulfate and chondroitin sulfate. Keratan sulfate moieties are found both in a keratan sulfate-rich region which is poor in chondroitin sulfate and in the major polysaccharide attachment region where one finds the majority of the chondroitin sulfate chains. Chondroitin sulfate moieties have molecular weights ranging in humans from approximately 5000 in disc tissue to approximately 15,000 to 20,000 in young articular cartilage. Keratan sulfate ranges in molecular weight from approximately 3000 to approximately 9000, being slightly larger in disc tissue than in articular cartilage. Keratan sulfate consists of a repeating sequence of the disaccharide, glucosamine-galactose. The degree to which each of these two sugars is sulfated varies with the source of the keratan sulfate. It is as yet unclear what the exact composition of the sequence of sugars which make up the linkage region linking the keratan sulfate molecules to the protein is.
During postnatal life, proteoglycan molecules change with respect to size and chemical composition. The changes include modifications in the size of the chondroitin sulfate chains as well as in the relative content of chondroitin sulfate, keratan sulfate, and oligosaccharide molecules covalently attached to the core protein. The glycosaminoglycan moieties, particularly keratan sulfate, have thus been the object of recent studies. For example, it has been shown that proteoglycans isolated from osteoarthritic cartilage have compositions that are similar to those of proteoglycans isolated from immature tissue. Chondrocytes (cartilage cells), in an attempt at tissue repair, apparently switch to the biosynthesis of immature proteoglycans that are poor in keratan sulfate. Keratan sulfate content has been found to relate to cartilage stiffness, and it has been suggested that low keratan sulfate proteoglycans are unsuitable for the load-bearing requirements of joints.
Despite the fact that keratan sulfate has been the subject of recent investigation as relates to cartilage tissue, relatively little is known about keratan sulfate. Not only has keratan sulfate not yet been fully characterized, but little is known about its breakdown, or more generally about its metabolism.
Nevertheless, it is known that keratan sulfate is quite tissue specific being naturally present only in cartilage, disc, and corneal tissue. Chondroitin sulfate, on the other hand, is not as tissue specific, being found in a variety of other connective tissues. Clearly, keratan sulfate is a more specific indicator of cartilage group tissue, its precursors and its breakdown products than is chondroitin sulfate.